Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative-working. A positive photographic image can be formed by producing a negative photographic image and then forming a second photographic image which is a negative of the first negative, that is, a positive image. A direct-positive image is understood in photography to be a positive image that is formed without first forming a negative image. Positive dye images which are not direct-positive images are commonly produced in color photography by reversal processing in which a negative silver image is formed and a complementay positive dye image is then formed in the same photographic element. The term "direct reversal" has been applied to direct-positive photographic elements and processing which produces a positive dye image without forming a negative silver image. Direct-positive photography in general and direct reversal photography in particular are advantageous in providing a more straightforward approach to obtaining positive photographic images.
A conventional approach to forming direct-positive images is to use photographic elements employing internal latent image-forming silver halide grains. After imagewise exposure, the silver halide grains are developed with a surface developer; that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a negative-working photographic element. The internal latent image-forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a comparatively slow rate, as compared to the internal latent image-forming silver halide grains not imagewise exposed. The result is a direct-positive silver image. In color photography, the oxidized developer that is produced during silver development is used to produce a corresponding positive, direct reversal dye image. Multicolor direct reversal photographic images have been extensively investigated in connection with image-transfer photography.
It has been found advantageous to employ nucleating agents in preference to uniform light exposure in the process described above. The term "nucleating agent" is employed herein to mean a fogging agent capable of permitting the selective development of internal latent image-forming silver halide grains which have not been imagewise exposed in preference to the development of silver halide grains having an internal latent image formed by imagewise exposure.
A favored class of nucleating agents is arylhydrazides. These nucleating agents can be incorporated in a developer solution or directly within a photographic element. Significant advantages have been realized by adsorbing arylhydrazide nucleating agents to the surface of internal latent image-forming silver halide grains. This permits small amounts of the nucleating agents to be employed, as compared with those which are nonadsorbed. However, this narrows the choice of arylhydrazide nucleating agents to those including an adsorption-promoting moiety.
Highly effective adsorbed arylhydrazide nucleating agents are the acylhydrazinophenylthioureas of Leone et al U.S. Pat. No. 4,030,925. These acylhydrazinophenylthioureas are characterized by the 3-position nitrogen atom of the thiourea moiety being mono-substituted.